First, a quick explanation: For a major El Niño event, the atmosphere and ocean have to join forces. The Pacific trade winds can actually reverse direction during strong El Niños, pulling reinforcing shots of warm water to the surface and initiating a global chain reaction of abnormal weather.

In the end, a big El Niño just never happened. And now it looks like even a small one is iffy. What gives?

On the National Oceanic and Atmospheric Administration’s official El Niño blog, Emily Becker writes that weaker events, like the one shaping up this year, are harder to predict. Even though seasonal climate forecasting has been quite good for decades, the majority of missed events have occurred more recently, since 2000, when weaker events have been the rule.

Seasonal climate forecasts have a tendency to focus on the telltale central Pacific warming signal that defines El Niño. But this year, the Pacific was warm pretty much everywhere, perhaps throwing the crucial atmosphere-ocean linkage necessary for a mature El Niño out of kilter.

When the ocean surface is warm all over, there’s no strong temperature gradient for the atmospheric component to build from.

And most importantly, for El Niño’s purposes, the gradient in sea surface temperature is not strong across the equator from Australia to South America, either. This gradient—from cool in the west to warm in the east—drives winds across the equator, which in turn causes a stronger temperature gradient, and so on.

In essence, a gradually warming Pacific Ocean is at once reducing our ability to predict Earth’s single most important seasonal climate phenomenon, and tampering with it as well. For forecasters, that means this year’s El Niño tease has been “rather frustrating.” It mirrors another flash-in-the-pan-and-fizzle just two years ago.